1. Field of the Invention
The present invention relates to a bias circuit and an amplifier that have a current limit function capable of limiting a bias current exceeding a normal range by varying a control voltage.
2. Description of the Related Art
In general, selecting a wireless communications scheme is a matter of selecting a digital modulation/demodulation scheme, and an appropriate scheme has been employed in consideration of frequency utilization efficiency. For instance, a quadrature phase shift keying (QPSK) scheme is employed for a mobile telephone using a code division multiple access (CDMA) scheme, and an orthogonal frequency division multiplexing (QFDM) scheme is employed in a wireless LAN conforming to the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards.
Wireless communications systems in which such wireless communications schemes are employed include power amplifiers for amplifying high frequency signals.
Here, in a system requiring linear amplification, a power amplifier having linear characteristics is required to amplify a transmission signal without distortion. Here, linear characteristics mean that the power of an output signal is amplified at a constant rate while the phase thereof is not changed, even in the case that the power of an input signal varies.
In addition, a technology which compensates for temperature so as to prevent a power amplification rate from significantly changing, even in the case that ambient temperature changes, is required.
In a typical power amplifier, a bias voltage generated by a bias circuit is supplied to a power amplifying element. The bias circuit may include a temperature compensation function.
However, a temperature compensation function in a typical power amplifier, performed by a transistor, may have limitations, such that bias current may increase due to an increase in temperature. In this case, the increased bias current may cause the temperature at the junction of the transistor included in a bias voltage generating unit to be increased, a so-called thermal runaway phenomenon.
Patent Document 1 relates to a power amplifier but does not disclose limiting a high bias current exceeding a normal range by varying a control voltage.